1. Field of the Invention
The present invention relates to a polishing apparatus and polishing method for polishing a substrate such as a semiconductor wafer, and more particularly to a polishing apparatus and polishing method for polishing a periphery of a substrate using a polishing tape.
2. Description of the Related Art
From a viewpoint of improving a yield in semiconductor fabrications, management of a surface condition in a periphery of a semiconductor wafer has recently been drawing attention. In semiconductor fabrication processes, a number of materials are deposited on a wafer repeatedly to form multilayer structures. As a result, unwanted materials and roughened surface are formed on a periphery which is not used for products. In recent years, it has become more common to transfer the wafer by holding only the periphery of the wafer. Under such circumstances, the unwanted materials could come off the periphery onto devices formed on the wafer during several processes, resulting in a lowered yield. Thus, it has been customary to polish the periphery of the wafer using a polishing apparatus to remove an unwanted copper film and the roughened surface.
FIG. 1 is a schematic view of a conventional polishing apparatus for polishing a periphery of a wafer. As shown in FIG. 1, this type of polishing apparatus has a wafer stage unit 120 configured to hold a wafer W, a polishing head 142 configured to press a polishing tape 141 against a periphery of the wafer W, a supply reel 145a for supplying the polishing tape 141 to the polishing head 142, and a rewind reel 145b for rewinding the used polishing tape 141. The supply reel 145a and the rewind reel 145b are rotated so as to supply and rewind the polishing tape 141. The supply reel 145a and the rewind reel 145b are fixed to a stationary member that is part of the polishing apparatus.
The polishing head 142 has a tape transfer mechanism 143 therein. This tape transfer mechanism 143 has a tape-transferring roller and a holding roller. The polishing tape 141 is interposed between the tape-transferring roller and the holding roller, while the tape-transferring roller is rotated to thereby transfer the polishing tape 141. The polishing tape 141 is pulled toward the polishing head 142 by the tape transfer mechanism 143. A travel direction of the polishing tape 141 is guided by guide rollers 157a, 157b, 157c, 157d, 157e such that the polishing tape 141 is placed in contact with the periphery of the wafer W. After contacting the periphery of the wafer W, the polishing tape 141 is rewound by the rewind reel 145b. The supply reel 145a and the rewind reel 145b are coupled to non-illustrated motors that provide the polishing tape 141 with a suitable tension. The polishing head 142 performs a linearly-reciprocating motion in vertical directions, as indicated by arrows in FIG. 1, to thereby provide relative movement between the polishing tape 141 and the wafer W, thus polishing the periphery of the wafer W.
FIG. 2 is a plan view illustrating mechanisms for the linearly-reciprocating motion of the polishing head 142. FIGS. 3A through 3D are views taken along line III-III in FIG. 2.
The polishing head 142 is fixed to one end of a movable arm 160, and a cam shaft 161 is provided at another end of the movable arm 160. The movable arm 160 is supported by a support arm 162 via linear guides 163. These linear guides 163 are configured to guide movement of the movable arm 160 so as to allow the movable arm 160 to move linearly relative to the support arm 162. A motor M for driving the cam shaft 161 is mounted on the support arm 162. This motor M has a rotational shaft coupled to the cam shaft 161 via pulleys p101 and p102 and a belt b100. The rotational shaft of the motor M and the cam shaft 161 are rotatably supported by bearings 164A and 164B mounted on the support arm 162. The cam shaft 161 has an eccentric shaft 161a which is off-centered with respect to a central axis of the bearing 164B. A cam 165 is mounted on a tip end of the eccentric shaft 161a. This cam 165 is fitted into a groove 166 of a U-shaped portion of the movable arm 160 (see FIG. 3A).
As the motor M is energized, the cam shaft 161 is rotated via the pulleys p101 and p102 and the belt b100. Since the eccentric shaft 161a is rotated eccentrically with its central axis deviating from the central axis of the bearing 164B, the cam 165 is also rotated eccentrically. As shown in FIGS. 3A through 3D, eccentric rotation of the cam 165 in the groove 166 causes the movable arm 160 to perform a linearly-reciprocating motion in vertical directions, thus causing the polishing head 142 on the tip end of the movable arm 160 to perform a linearly-reciprocating motion in vertical directions.
The tape transfer mechanism 143 is operable to transfer the polishing tape 141 from the supply reel 145a to the rewind reel 145b at a constant speed. On the one hand this tape-transferring speed is very low, and on the other hand a speed of the polishing head 142 reciprocating linearly is very high. Therefore, in the tape supplying and rewinding operations of the supply reel 145a and the rewind reel 145b, which will be discussed later, the tape-transferring speed is practically negligible as compared with the speed of the reciprocating polishing head 142.
FIGS. 4A through 4C are views illustrating the operations of the polishing head 142, the supply reel 145a, and the rewind reel 145b. In FIGS. 4A through 4C, reference symbols A and B represent certain points on the polishing tape 141 attached to the supply reel 145a and the rewind reel 145b. 
FIG. 4A shows a state in which a center Ch of the polishing head 142 is aligned with a thickness center Cw of the wafer W (which will be hereinafter referred to as simply a wafer center). The wafer center Cw is fixed in position during polishing. On the other hand, the polishing head 142 performs the linearly-reciprocating motion across the wafer center Cw.
FIG. 4B shows a state in which the polishing head 142 is linearly moved downwardly from the wafer center Cw. During this downward movement, the polishing tape 141 is pulled from the supply reel 145a and the rewind reel 145b, and the points A and B on the polishing tape 141 are moved downwardly, as shown in FIG. 4B. As a result, the supply reel 145a and the rewind reel 145b are rotated through a certain angle corresponding to a length of the polishing tape 141 that has been pulled out.
FIG. 4C shows a state in which the polishing head 142 is linearly moved upwardly from the wafer center Cw. During this upward movement, the polishing tape 141 is rewound by the supply reel 145a and the rewind reel 145b, and the points A and B on the polishing tape 141 are moved upwardly, as shown in FIG. 4C. As a result, the supply reel 145a and the rewind reel 145b are rotated through a certain angle corresponding to a length of the polishing tape 141 that has been rewound. In this manner, in accordance with a change in distance between the polishing head 142 and the supply and rewind reels 145a and 145b, the supply reel 145a and the rewind reel 145b repeat supply and rewind of the polishing tape 141, as shown in FIGS. 4B and 4C.
However, as the supply reel 145a and the rewind reel 145b repeat supply and rewind of the polishing tape 141, the tension of the polishing tape 141 would change when the supply and rewind operations are switched, resulting in unstable polishing performance. In particular, as the reciprocating motion becomes faster, an excessive tension could be exerted on the polishing tape 141 when the supply and rewind operations are switched. As a result, the polishing tape 141 would be cut or stretch (i.e., permanent strain). On the other hand, the tension of the polishing tape 141 could be zero when the supply and rewind operations are switched. As a result, the polishing tape 141 slackens, and a desired polishing performance cannot be obtained.
If the reciprocating speed of the polishing head 142 can be increased, a removal amount of materials per unit time (i.e., a polishing speed or a removal rate) can be increased. Therefore, the polishing performance of the polishing apparatus can be improved. However, because of the above-described problems, the reciprocating speed of the polishing head 142 cannot be increased so high. In order to solve such problems, the supply reel 145a may be incorporated in the polishing head 142. In this case, however, a motor for providing the polishing tape with a tension becomes heavy. Moreover, if a long polishing tape 141 is to be used, the supply reel 145a is required to be large. This means that the polishing head 142 and the drive mechanisms for the reciprocating motion are required to be large in size.